1. Field of the Invention
The present invention relates to a method for the preparation of .alpha.-L-aspartyl-L-phenylalanine methyl ester (hereafter abbreviated as .alpha.-APM). .alpha.-APM is a peptide sweetener which exhibits a sweetness about 200 times that of sucrose.
2. Discussion of the Background
.alpha.-APM has been widely used as a dieting sweetener in recent years because of its high quality sweetness and low calorie content. It is thus expected that its demand over the world will exceed 10,000 tons through 1995.
The following examples of methods for preparing .alpha.-APM on an industrial scale are known:
(1) A method for preparing .alpha.-APM which comprises binding an N-substituted aspartic anhydride to L-phenylalanine methyl ester in an organic solvent, splitting the substituent off in a conventional manner (U.S. Pat. No. 3,786,039), contacting the formed .alpha.-APM including impurities with a hydrohalogenic acid to obtain .alpha.-APM hydrohalide and then neutralizing the hydrohalide;
(2) A method which comprises converting .alpha.-L-aspartyl-L-phenylalanine in a solvent mixture of water, methanol and hydrochloric acid into the corresponding methyl ester to form .alpha.-APM hydrochloride, and neutralizing the hydrochloride to give .alpha.-APM (Japanese Patent Application Laid-Open No. 53-82752); and
(3) A method which comprises condensing an N-substituted aspartic acid with phenylalanine methyl ester in the presence of enzyme and then removing the substituent (Japanese Patent Application Laid-Open No. 55-135595).
In the methods (1) through (3) described above, .alpha.-APM is eventually crystallized using a solvent which dissolves .alpha.-APM, such as water at a high temperature or a hydrated lower alcohol. The crystals are isolated and dehydrated using an apparatus for solid-liquid separation, such as a centrifugal machine, and then dried to obtain the final product.
Crystallization methods include cooling crystallization, neutralization crystallization and concentration crystallization. However, .alpha.-APM decomposes to diketopiperazine (and other compounds) at high temperatures, such as those often employed in concentration crystallization. As a result, cooling crystallization is preferred when considering the temperature stability of .alpha.-APM.
Cooling crystallization is generally carried out using a stirring crystallizer having a heat transfer surface for cooling, or a crystallizer equipped with a heat exchanger in an external circulation system. When cooling crystallization is performed using a crystallizer accompanied by forced fluidization, such as stirring or external circulation, fine needles of .alpha.-APM are usually obtained. Such fine needles have poor properties for solid-liquid separation, such as a poor filterability or dewaterability. As a result, conventional methods (for example, a method for crystallization which comprises cooling a hot aqueous solution of APM by indirect heat transfer with a coolant at a low temperature) requires a large filtering area for solid-liquid separation.
In addition, such methods result in crystals precipitating in large amounts upon crystallization. The precipitated crystals adhere to the cooling surface, thus markedly reducing the cooling efficiency.
In order to efficiently cool a crystallizing solution, scales (adhered crystals) must be removed from cooling surface. Continuous crystallization generally provides high productivity on an industrial scale, however, if scaling occurs, it is necessary to stop operation often. Then productivity is unavoidably reduced. Although scaling does not cause a serious problem in an experiment for a short period of time using a glass vessel, such as those frequently used in a research laboratory, scaling often creates a serious problem over longer periods of time using a metallic apparatus, such as those used for continuous crystallization on an industrial scale.